This invention relates to an arrangement for turning on and turning off a power transistor, comprising:
a switching transistor having a first and a second main electrode which constitute a main current conductance path of the switching transistor, and a control electrode for a control signal for controlling the conductance of the main current conductance path of the switching transistor; PA0 a control device comprising a control amplifier having an input which is coupled to a switching signal terminal for connection to a switching signal and an output for applying the control signal to the control electrode of the switching transistor; PA0 a thyristor having a first main electrode and a second main electrode which constitute a main current conductance path of the thyristor, which path is coupled to the control electrode of the switching transistor, and a first trigger gate and a second trigger gate for receiving a first trigger signal and a second trigger signal, respectively; PA0 measuring means for generating a measuring signal which is proportional to a current flowing through the main current conductance path of the switching transistor; and PA0 comparison means for comparing the measuring signal with a reference signal and for applying the second trigger signal to the second trigger gate of the thyristor in response to the comparison of the measuring signal and the reference signal. PA0 a further control amplifier having an input which is coupled to the switching signal terminal and an output which is coupled to the first trigger gate of the thyristor for applying a further control signal to the first trigger gate; and PA0 delay means for delaying the control signal of the control amplifier with respect to the further control signal of the further control amplifier.
An arrangement of this type is known from U.S. Pat. No. 5,006,949, FIG. 1. In this known arrangement the switching transistor is an N-channel MOS transistor whose second main electrode or drain is connected to the positive terminal of a power supply source via a load and whose first main electrode or source is connected to the negative terminal of the power supply source via a measuring resistor. A measuring voltage is produced across this measuring resistor, which voltage is proportional to the current flowing through the load and through the main current conductance path of the switching transistor as soon as the control amplifier applies a positive going control signal to the control electrode or gate of the switching transistor at the command of the switching signal. A thyristor is arranged across the series arrangement of the measuring resistor and the gate-source junction of the switching transistor, which thyristor is triggered by means of a trigger signal as soon as the current through the main current conductance path of the switching transistor exceeds a given value. To this end the second trigger gate, or cathode gate, of the thyristor is connected by means of a current-limiting resistor to the junction point of the measuring resistor and the source of the switching transistor, while the second main electrode, or cathode, of the thyristor is connected to the junction point of the measuring resistor and the negative terminal of the power supply source. The first main electrode, or anode, of the thyristor is connected to the gate of the switching transistor and the first trigger gate of the thyristor, or anode gate, is connected to the anode by means of a damping resistor so as to damp the trigger sensitivity of the thyristor. Upon triggering, the thyristor short-circuits the control signal at the gate of the switching transistor via the main current conductance path constituted by the anode and cathode. Thus, the switching transistor is protected from too large currents which may occur, for example, upon a short-circuit in the load. The thyristor can be considered to be a bipolar semiconductor element which is composed of a PNP transistor and an NPN transistor. The emitter, base and collector of the PNP transistor are connected to the anode, anode gate and cathode gate, respectively, of the thyristor, and the emitter, base and collector of the NPN transistor are connected to the cathode, cathode gate and anode gate, respectively, of the thyristor. As soon as the measuring voltage across the measuring resistor exceeds the base-emitter threshold voltage of the NPN transistor, the thyristor is triggered.
In the known arrangement the thyristor is triggered only in the case of emergency. The switching transistor is normally turned off also at the command of the switching signal. In an adapted form the known arrangement is also usable in switched-mode power supplies in which the load is constituted by the primary winding of a transformer. In this case the switching transistor is first turned on so that the current through the primary winding increases and is subsequently turned off again as soon as the current through the primary winding exceeds a given value. This value is variable and is controlled by means of a system rendering the voltage supplied by the switched-mode power supply independent of, inter alia, the load. The switching transistor is turned on by applying a control signal to the control electrode of the switching transistor at the command of the switching signal having the character of a set signal. The switching transistor is turned off by triggering the thyristor with the cathode trigger signal which has the character of a reset signal. To be able to control the turn- off value of the current through the switching transistor in the known arrangement, the measuring resistor has to be variable. In view of the generally small ohmic value of the measuring resistor, such a solution is impractical and difficult to integrate on a semiconductor body. By means of a sense transistor the current through the switching transistor can be scaled to a smaller value and converted to a measuring voltage by means of a correspondingly larger variable measuring resistor. Such variable measuring resistors are suitable for integration on a semiconductor body, but this is still a complicated solution. However, variable current sources, voltage current converters and current mirrors are electronic components which can be integrated relatively easily. For example, the current through the sense transistor can be compared with a variable reference current by means of a current mirror. As soon as the current through the sense transistor is larger than the reference current, a current is generated which may be used as a trigger current for the cathode gate of the thyristor. Since the trigger signal for the cathode gate now has the shape of a current and is supplied by a source having a relatively high impedance, a problem presents itself. A parasitic capacitor having a value which is determined, inter alia, by the sum of the collector-base capacitances of the PNP and NPN transistors is present between the anode gate and the cathode gate of the thyristor. In the known arrangement this parasitic capacitor is charged via the series arrangement of the damping resistor between the anode and the anode gate and the current-limiting resistor between the cathode gate and the measuring resistor during the period when the control voltage at the gate of the switching transistor is positive. The damping resistor and the current-limiting resistor have relatively small values and the voltage drop across these resistors due to charging the parasitic capacitor is too small to trigger the thyristor. However, if the impedance from which the cathode gate is driven is very large, as is the case when triggering from a current source, the charge current will find a path to the base of the NPN transistor. The NPN transistor becomes conductive and will amplify the parasitic capacitance which is present at the anode gate by a factor as a result of the Miller effect. The amplification is dependent on the current gain (beta) of the NPN transistor. This has a dual result. On the one hand the output of the control amplifier is capacitively loaded to a larger extent when the switching signal for the switching transistor is generated and may consequently exceed the output current which can be maximally supplied by the control amplifier. On the other hand the charge current of the amplified parasitic capacitance will generate a larger voltage drop across the damping resistor so that the thyristor may be triggered inadvertently. A solution could be to reduce the damping resistor by a factor of beta so as to avoid unwanted triggering when the switching signal occurs. However, this leads to a thyristor which is triggerable with great difficulty. At the instant when the switching transistor is turned off, the thyristor will have to be triggered. It is no problem to render the NPN transistor conducting. The collector current of the NPN transistor substantially flows through the damping resistor and is supplied by the output of the control amplifier. This amplifier must be able to supply a large current in order to generate a base-emitter threshold voltage across the small damping resistor so as to render the PNP transistor also conducting and thus trigger the thyristor. It is not a satisfactory solution to give the damping resistor a very large value or to omit it because the thyristor will then be much too sensitive and will be triggered inadvertently whenever the positive switching signal occurs.